Orientation of the tetranuclear manganese cluster and tyrosine Z in the O₂-evolving complex of photosystem II: An EPR study of the S₂Y(z)· state in oriented acetate-inhibited photosystem II membranes

Inhibitory treatment by acetate, followed by illumination and rapid freezing, is known to trap the S₂Y(z)· state of the O₂-evolving complex (OEC) in photosystem II (PS II). An EPR spectrum of this state exhibits broad split signals due to the interaction of the tyrosyl radical, Y(z)·, with the S = 1/2 S₂ state of the Mn₄ cluster. We present a novel approach to analyze S₂Y(z)· spectra of one-dimensionally (1-D) oriented acetate- inhibited PS II membranes to determine the magnitude and relative orientation of the S₂Y(z)· dipolar vector within the membrane. Although there exists a vast body of EPR data on isolated spins in oriented membrane sheets, the present study is the first of its kind on dipolar-coupled electron spin pairs in such systems. We demonstrate the feasibility of the technique and establish a rigorous treatment to account for the disorder present in partially oriented 1-D membrane preparations. We find that (i) the point-dipole distance between Y(z)· and the Mn₄ cluster is 7.9 ± 0.2 Å, (ii) the angle between the interspin vector and the thylakoid membrane normal is 75°, (iii) the g(z)-axis of the Mn₄ cluster is 70°away from the membrane normal and 35°away from the interspin vector, and (iv) the exchange interaction between the two spins is -275 x 10^-4 cm^-1, which is antiferromagnetic. Due to the sensitivity of EPR line shapes of oriented spin-coupled pairs to the interspin distance, the present study imposes a tighter constraint on the Y(z)-Mn₄ point-dipole distance than obtained from randomly oriented samples. The geometric constraints obtained from the 1-D oriented sample are combined with published models of the structure of Mn- depleted PS II to propose a location of the Mn₄ cluster. A structure in which Y(z) is hydrogen bonded to a manganese-bound hydroxide ligand is consistent with available data and favors maximal orbital overlap between the two redox center that would facilitate direct electron- and proton-transfer steps.